Ebola's initial flu-like symptoms can lead to external haemorrhaging from areas such as eyes and gums, and internal bleeding which can lead to organ failure. Patients have a better chance of survival if they receive early treatment.

Nurse 'skipped quarantine'

The outbreak began in Guinea in February and has killed 377 people in the country.

It has since spread to Liberia, Sierra Leone and Nigeria, which have all declared a national health emergency, claiming a total of 1,069 lives by Wednesday, according to WHO.

Ebola virus disease (EVD)

Symptoms include high fever, bleeding and central nervous system damage

Fatality rate can reach 90% - but the current outbreak is about 55%

Incubation period is two to 21 days

There is no vaccine or cure

Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery

Fruit bats are considered to be virus' natural host

Guinea's President Alpha Conde, in a statement read out on state radio, said people who had been in contact with Ebola victims were "formally banned from leaving their homes until the end of their surveillance period".

Anyone who contravened the measures would be considered "a threat to public health and will face the might of the law", the statement said.

In Nigeria, where a third person died of Ebola on Tuesday, Health Minister Onyebuchi Chukwu said a doctor had been infected, bringing to 11 the number of confirmed cases in Africa's most populous state.

The doctor had initially treated Liberian government employee Patrick Sawyer, who brought the disease to Nigeria's main city, Lagos, last month and died in quarantine, Mr Chukwu said.

He added that a nurse who also caught Ebola from Mr Sawyer was in an isolation ward in Lagos - the only place in Nigeria so far affected by Ebola.

Bodily fluids spread the virus

On Wednesday, Nigeria's information minister said the nurse had been brought back to Lagos after disobeying medical instructions by travelling to the eastern city of Enugu.

Labaran Maku said 21 people who had contact with her in Enugu were "being watched" but the health minister said the number was six.

In other developments:

Nigeria has withdrawn from the Youth Olympics in China, due to start on Saturday, after Chinese officials quarantined its athletes

Korean Air Lines says it will stop flying to Kenya from 20 August after WHO identified it as a "high-risk" country for the spread of Ebola.

Kenya Airways has rejected pressure from the local medical association to suspend all flights to Ebola-hit states, saying the WHO has not recommended a ban on air travel.

This is the first time West Africa has been affected by Ebola - previous outbreaks have affected East and Central Africa.